Histone acetylation is most usually associated with the activation of gene transcription, as the modification loosens the interaction of the DNA and the histone octomer by changing the electrostatics. In addition to this physical change, specific proteins bind to acetylated lysine residues within histones to read the epigenetic code. Bromodomains are small (about 110 amino acid) distinct domains within proteins that bind to acetylated lysine resides commonly but not exclusively in the context of histones. There is a family of around 50 proteins known to contain bromodomains, and they have a range of functions within the cell.
The BET family of bromodomain containing proteins comprises 4 proteins (BRD2, BRD3, BRD4 and BRD-T) which contain tandem bromodomains capable of binding to two acetylated lysine residues in close proximity, increasing the specificity of the interaction. Recent research has established a compelling rationale for targeting BRD4 in cancer. BRD4 remains bound to transcriptional start sites of genes expressed during the entry into the G1 phase of the cell cycle, and is functioning to recruit the positive transcription elongation factor complex (P-TEFb), resulting in increased expression of growth promoting genes (Yang and Zhou, Mol. Cell. Biol. 28, 967, 2008). Importantly, BRD4 has been identified as a component of a recurrent t(15;19) chromosomal translocation in an aggressive form of human squamous carcinoma (French et al., Cancer Res. 63, 304, 2003). Such translocations express the tandem N-terminal bromodomains of BRD4 as an in-frame chimera with the NUT (nuclear protein in testis) protein, genetically defining the so-called NUT midline carcinoma (NMC). Functional studies in patient-derived NMC cell lines have validated the essential role of the BRD4-NUT oncoprotein in maintaining the proliferation and the differentiation block of these malignant cells. In addition, BRD4 has been identified as a critical sensitivity determinant in a genetically defined AML mouse model (Zuber et al., Nature 2011 478(7370):524-8). Suppression of BRD4 led to robust anti-leukemic effects in vitro and in vivo, accompanied by terminal myeloid differentiation. Interestingly, BRD4 inhibition triggered MYC down-regulation in a broad array of mouse and human leukemia cell lines examined, indicating that small molecule BRD4 inhibitors may provide a means to suppress the MYC pathway in a range of AML subtypes.
Finally, the other family members of the BET family have also been reported to have some function in controlling or executing aspects of the cell cycle, and have been shown to remain in complex with chromosomes during cell division—suggesting a role in the maintenance of epigenetic memory (Leroy et ai, Mol. Cell. 2008 30(1):51-60).
Examples of bromodomain inhibitors are benzodiazepine derivatives, disclosed in WO2011/054553, and imidazo[4,5]quinoline derivatives, disclosed in WO2011/054846.
Thus, there is the need to provide BRD4 inhibitors useful for the prevention and/or treatment of diseases characterized by excessive or abnormal cell proliferation, such as cancer.